In a series of raging tweets, the United States president urged Mexico to "stop this onslaught" and prevent the would-be immigrants from entering the US.
The latest focus is on more than 2,000 Hondurans who left last Saturday from the city of San Pedro Sula.
On Wednesday, Mexican officials said those in the Honduran caravan with proper documentation could enter the country and those without it would have to apply for refugee status or face deportation.
A caravan of thousands of migrants continues to slalom its way through Central America in hopes of reaching a safe haven beyond the southern USA border. 20,000 of these migrants die or disappear along the way, many of them at the hands of criminal gangs, according to a report by the Mesoamerican Migration Movement.
Mr Trump has previously threatened to cut Honduran aid. The UN's assistance is necessary "to avoid people being deceived by global criminal organizations dedicated to human trafficking", the statement said.
Now there are as many as 4,000 people - men, women and children streaming across Guatemala and hoping to traverse Mexico and reach the U.S.
But it added that all cases must be processed individually, suggesting Mexican authorities have no intention of letting the migrants simply cross the border en masse without going through standard immigration procedures.
Mexico's ambassador to the United States, Geronimo Gutierrez, retweeted a news release announcing that Mexico has requested help from the U.N. High Commissioner for Refugees (UNHCR).
United States officials have already began expressing concerns about the caravan, which also prompted angry tweets from President Trump.
Trump's comments come as he has been urging fellow Republicans to make immigration a central issue in the closing weeks of their midterm election campaigns and blaming Democrats for his inability to pass immigration legislation in the GOP-controlled Congress.
It has also prompted a slew of new rulings about asylum law from Attorney General Jeff Sessions, and the introduction of a "zero tolerance" policy for border crossers that led to family separations. They served up beef broth, rice, tortillas and coffee to migrants passing on foot who paused for refreshment at the impromptu oasis.
Current Foreign Relations Secretary Luis Videgaray was also sanguine and viewed things through the lens of US politics.